Mold



Mi 3, 1949. c. L. SCHEER ET AL 2,464,157

MOLD

Filed June 28, 1944 3nnentors '5 attorney Patented Mar. 8, 1949 MOLD Charles L. Scheer, Haddonfield, N. J and Willis Fleisher, Jr., Elkins Park, Pa., assignors to Radio Corporation of America, a corporation of Delaware Application June 28, 1944, Serial No. 542,482

3 Claims.

This invention relates to the art of molding or casting plastic materials of the type which shrink as they congeal from their plastic or fluid state to the normally solid form. While not limited thereto, the invention will be described as applied to the casting of such materials for use as insulating embedments for electrical units such, for example, as capacitors, radio coils, transformers, resistors or the like.

Among the materials heretofore used as insulating embedments are certain mineral waxes, (e. g. paraffin), vegetable oils (e. g. hydrogenated castor oil), waxes (e. g. cerese, carnauba) and synthetic resins (e. g. esters of acrylic and alkacrylic acids, polystyrene, etc.). These and numerous other insulating materials adhere to the surface of the mold cavity and, at the same time, shrink in volume as they congeal from their plastic or liquid state to their ultimate solid form. This, usually, gives rise to bubbles and other voids Within the embedment. Such voids may contain small quantities of moisture or gas which, when ionized during the operation of the embedded device, provides a leakage path of low electrical resistance through the unit and shortens its life.

Accordingly, the principal object of the present invention is to provide an improved, molded, cast or potted structure and one free from bubbles, cracks and other imperfections.

Another and important object of the invention is to provide an improved method of casting, molding or potting plastic materials of the general character described, and one which may be practiced with a mold or matrix of standard form and dimensions.

Another and related object of the invention is to provide a simple, inexpensive and trouble-free method of treating a metal matrix or mold to facilitate the removal therefrom of the object formed therein.

Still another object of the present invention is to provide a novel and reliable method of applying a metallic coating to the surface or surfaces of a molded object during the molding operation.

The foregoing and other objects are achieved in accordance with the invention by providing-the cavity of the mold or matrix with a weakly adherent metallic lining, the strength of adhesion of the lining to the walls of the cavity being less than the strength of adhesion of the molding material to the lining. Thus, when the molding material shrinks as it congeals from its plastic state to its solid form, the lining is stripped (without the use of any auxiliary mechanical force) from the Walls of the cavity and adheres tightly to the adjacent surface of the said material, which is thereby prevented from being torn apart as it shrinks. Thus, the method of the invention provides a molded or cast object having an adherent metallic coating on the outer surface thereof.

This coating is free from imperfections and, if desired, may be allowed to remain on the object either for utilitarian or decorative purposes. However, when necessary or desirable, the metalliclining or coating which adheres to the casting may subsequently be removed by immersing it in a solvent to which the molding material per se is immune.

Other objects and advantages together with certain preferred details of construction and procedure will be apparent, and the invention itself will be best understood by a reference to the following specification and to the accompanying drawing, wherein:

Figure 1 shows, in sectional elevation, one form of an apparatus which may be used in applying a film-like metal lining to a mold, in accordance with the invention,

Figure 2 is a partly broken away exploded view, in perspective, of a mold of standard form and dimensions having a weakly adherent metallic lining on the cavity-surfaces thereof, and

Figure 3 is a molded capacitor constructed in accordance with the principle and method of the invention.

In the drawing, wherein like reference characters designate the same parts in all figures, I designates generally a more or less conventional open-top mold comprising a base 3 and two upstanding side walls or plates 5 and 1 which are adapted to be joined in juxtaposition on the base 3, as by means of screws 9 which are received in suitably tapped holes l with which the said separable parts are provided.

The mold which has been selected for purposes of illustration is designed for the manufacture of pairs of molded capacitors similar to the one shown at l 3, Figure 3 and, to this end, is provided with two cavities I 5 and I1 and with a number of paired horizontal slots l9 and pins 2| for support-. ing the armatures 23 and 25 of the said capacitors during the embedment operation.

In carrying the invention into eifect, the side walls and, preferably, the base of each cavity are provided with a thin lining or coating 2'! constituted of metal (e. g. silver), in a finely divided state. As previously pointed out, it is important that the strengh of adhesion of the lining 21 to the walls of the cavity or cavities I5 and I! be less than the strength of adhesion of the molding material to the lining, and the invention may be said to reside, in part, in the provision of such a weakly adherent lining, and one which, unlike a discrete foil lining, can be easily removed fromthe casting by washing it in a suitable solvent.

In this connection it may be mentioned that an entirely satisfactory lining. for the; mold cannot ordinarily be achieved by the conventional Rochelle salts method. or by the Brasheer method.

Linings applied to a mold by chemical GGDOSitlOHiKIOL-" methods and by spraying also proved unsatisfactory, either because of difficulties in achieving a lining of uniform thickness orlbecause they ad-rn hered too tightly to the metal.wvall'stof theqcavityr It has been found that a metal-coating:havingr i,

the required low strength of adhesion can be applied to the interior surfaces ofqa moldor ma;- trix by thermal evaporation of a pellet of silver or other metal (e. g. copper, iron or aluminum); in-

vacuo. In the usual thermal evaporation method the object upon which the metal is, to .be deposited; is, first thoroughly cleansed and. subsequently;- bombarded or heatedto drive 01f occluded-airs. This procedure, however,- may result inytoo strong;

a bond and, in carrying the presentinvention -into5- efitect, it is preferable to omit those steps in, the." conventional process which are calculated .to drive}: the occluded air 'or other egas from'the surfaces uponrwhich the metal lining is to be applied;- Ac.-.

cordingly, referringnow to Fig. 1, in-applyingthe-.-

metallic lining to the inner surfaces of themold. it :is: merely necessary 'to place the ;mold,v or the separate parts thereof, in an evacuable chamber-- 3! containing a refractorymetal filament, 33 and a si1ver,(or other metal) pellet-,35', and to-evap 35K crate the-silver by energizing the filament. Ap parentlyg'the layer of occluded lair adjacent to. the. metal surface-of the cavity, andlupongwh-ich the evaporated metal is deposited, ensures -therequired relatively low strengthof adhesion.

When the device to be potted comprises an". electrical unit the molding material will ordinate: ily be selected with a careful regard .toithepower 1 factor, resistance and other electrical character;

istics: required. Polystyrene; is commonly, em-- t;

ployedras the embedment materialfor. electric:v capacitors and the-present. invention lends-itself; readily to the use of suchmaterial. Thus,-.- when the'now metallically lined. mold, shown inzthe.

drawing, has been assembled with the capacitor; 50,:

armatures 23 and 25in position on the -pins,-2l .in

the slots l9,'*.it is emerely necessary to' pour or: otherwise insert the monomeric styrene (or other-1' polymerizable material) into-the cavitiesthrougha the -top of the moldand to polymerize it incsitua by'anyof theusual polymerizingmethods.v As. previously brought out, the molding compound; shrinks as it 'congeals from its plastic to its. solid. form and this shrinking action causesthe-metal. liningto be stripped from .the walls vofthe cavity- (without theuseof any auxiliary mechanicalz,

force) and to apl rearv asa'coatingyofpthe. surfaces. of -the'now polymerized; congealed or ;moldedaobl-,- ject. This linings, or coating, on 1 they-otherwise finished product may be removed, inewhole onin. 53 23139851? While as previously brought out, it is not necessary, nor indeed desirable, to drive off occluded gas from the metal walls of the cavity before the plating operation, it will be understood that the usual precautions should be taken against contaminating the mold surfaces with dirt and greases It w-lll. inow-:. be apparent that s the 1 present invention provides a simple and reliable method ofticasting or molding plastic materials and one which, in its practice, ensures a finished structure free from bubbles, cracks and other imperfectrons,

Whatyisclaimedcis:

1. A matrix'for: molding material of the type which shrinks asit'congeals from its plastic to its solid form, said. matrix comprising a metallic structure having a smooth cavity therein, said cavity having a weakly adherent metallic silver lining which has been thermally evaporated upon the-wal-lcf saicl'cavity by; placing; the matrix man evacuable chamber-wand thermally evaporating a metal .on: the cavity wall,-..whereby the-strength of'i adhesion ,ofisaidmetallic .liningto the .Wall of the cavity iswlessthan"the -strength of the adhesion:-

of ethe molding material .to "the; metallic dining:

2; A matrix :for molding-a material. of the type which. shrinks as it congealshfrom its: plastic states to its solid form, said matrixicomprising; azmetab licstructure having a smooth :cavity therein, said cavity having a -Weak1y adherent metallic silver: film. which has zbeen thermally: evaporated on the walla-of l-saidcavity while placed in an evacuated:

chamber whereby the strength of I adhesion, of said; .metallic lining -.to"the -'Wall :of; the cavity 'isless. than: the strength of the adhesion of the molding:

material .tc theimetailic lining;

3. A:- matrix formoldingaa material of the type" which shrinksas it congeals fromits plastic state:- 4unto-itssolidzform,said-:matrixcomprising a metallic structureihaving a smooth cavity therein, said cavityhaving? a=- weakly; adherent metallic film: which-has been thermally evaporated on the wall 1 ofsaidcavitywhile placed'in. an .evacuated chain- I ber whereby theestrength .of adhesion of said metalliclzliningtor-the. wall .ot .the. cavity is less thanthe strength of the; adhesion'of .the moldin material. to.-;the-- metallic :liningg CHARLES -L.. SCHEERZ. WILLISaFLEISHER, J R.

REFERENCES: CITED The followingrefe'rences' are of record in the fil'e of this patent:

UNITED :STATES- PATENTS Number Name Date 43,666 Bin-chardt' Aug; 2, 1864' 85,529" Laughlin" Jan. 5, 1869'- 2,079,784 Williams May 11, 1937 2}-16(5,205 Andersomet al-- July 18; 1939 2,237,328 Birdseye-"- Apr. 8, 19412 2,259,524- 2 Kistl'erz- Oct. 21, 1941' 2,274,444 Freedi" Feb; 24,- 1942 Bradshaw Mar. 16,1943 2,342,037 Cremert; Feb-l5, 1944 

